The present invention relates generally to a method and apparatus for the removal of stored energy from a storage phosphor screen and, more particularly, to such a method and apparatus for accelerated removal of energy stored within a reusable photostimulable storage phosphor screen which has previously been exposed to radiation to allow for prompt reuse of the screen in filmless radiography.
It is well known to use photostimulable storage phosphor screens (hereinafter referred to as a "phosphor screen") for performing filmless radiography. Some such phosphor screens are created by applying a coating of a phosphor layer onto a thin, flexible, rugged substrate, generally formed of a polymeric material. Such substrates are generally rectangularly shaped in top plan view and have a thickness in the range of 0.1 mm to 30 mm (0.004"-1.181"). The substrates are typically made of acrylic or MYLAR.RTM. although other polymeric materials may be employed. Although the material used to make the substrate, the size of the substrate and even the shape of the substrate may vary from application to application, rectangularly shaped polymeric substrates are generally preferred and such substrates are generally available in several different sizes, including 14 inches.times.17 inches, 7 inches by 10 inches, and 7 inches.times.17 inches, or any size in between.
The phosphor coating layer may be applied to the substrate using a variety of processes including creating a fine powder of the mixed phosphor elements or components and, thereafter, applying the powder generally evenly over one principal surface of the substrate and securing the powdered phosphor components to the substrate using a suitable binder, adhesive, or the like.
In the preferred embodiment of the present invention, as described in greater detail hereinafter, the components of the phosphor powder include strontium sulfide doped with cerium and samarium (SrS:Ce,Sm). Other components, such as CaS:Ce,Sm; SrS:Eu,Sm; CaS:Eu,Sm; or components having the general composition Ca.sub.x,Sr.sub.1-x :R,Sm, where R is Ce or Eu, could be used if desired. Phosphor screens of this type are commercially available from Liberty Technologies, Inc., of Conshohocken, Pa. Although a single type of phosphor screen having the above-described phosphor components will be discussed throughout the present application, it should clearly be understood that the principles involved with the present invention are not limited to a particular size, shape, or type of phosphor screen nor are they limited to particular phosphor components of the phosphor screen.
The processes employed for creating and, thereafter, "reading" radiographic images using a phosphor screen of the type described above are also generally well known in the art. In general, a phosphor screen (having negligible stored energy) is positioned adjacent to a product, device, person or item (hereinafter referred to as an "item") for which an image is desired, and the item and the phosphor screen are exposed to radiation from a radiation source positioned in such a manner that at least some of the radiation passes through the item before exposing the phosphor screen. The phosphor screen absorbs energy from the received radiation at varying levels and, depending upon the structure, material, and other aspects of the item, a latent image of the item is created on the phosphor screen through a well known process known as "electron trapping". Typically, prior to the aforementioned radiographic exposure, the phosphor screen is first placed in a special cassette or other packaging device which prevents the phosphor screen from being exposed to ambient light that could detrimentally affect the latent image of the item stored on the phosphor screen. An intensifier, comprised of a thin sheet of lead, copper or some other metal, may be positioned between the item and the phosphor screen to enhance the quality of the latent image created on the phosphor screen when high radiation energies are employed.
After creation of the latent image on the phosphor screen, the phosphor screen is "read", typically by a laser scanner and digitizer using a photostimulated luminescence process which is generally well known in the art. In the reading process, the entire phosphor screen is scanned, in accordance with a predetermined scanning pattern, by a high resolution near-infrared laser having wavelengths between 750 nanometers to 1,600 nanometers, with a peak at about 1,000 nanometers and preferably at a wavelength of about 1,000 nanometers. The laser scan has the effect of stimulating or releasing trapped electrons. The stimulation and release of the trapped electrons causes visible luminescence to be emitted from the phosphor screen in proportion to the energy level stored at specific locations on the phosphor screen (i.e., pixels). The intensity of the emitted luminescence for each area or pixel of the phosphor screen is electronically measured, utilizing a light sensitive device such as a photomultiplier tube, digitized and stored in a computer memory as a function of the laser position on the phosphor screen, thereby creating a gray scale image. Once stored within the computer memory, the digitized data representative of the latent image read from the phosphor screen may be recalled and displayed, typically on a high resolution monitor, for analysis or may be printed for later review and analysis, including trend analysis.
The above-described filmless radiography process is generally well known and equipment for performing the process is generally available from manufacturers including Liberty Technologies, Inc., of Conshohocken, Pa. In general, the laser used to stimulate the trapped electrons for release of the latent image from the phosphor screen is scanned very fast so that the image data is acquired fast. Typically, the intensity of the laser is between 1 and 500 milliwatts and the size of the laser spot is 25-250 .mu.m with the scan speed being between 5 and 500 microseconds per pixel. Thus, only a fraction of the total of the energy stored within the phosphor screen to create the latent image is released, depending upon the amount of laser energy employed. As a result, a substantial amount of energy, including the latent image, remains stored in the phosphor screen once the image reading process has been completed and the digital image data of the radiographed item has been stored in the computer memory. Because phosphor screens of the type described above are relatively expensive to produce, it is desirable to have the ability to reuse such phosphor screens multiple times. A phosphor screen which is still storing a substantial amount of energy, including a latent image of a previously radiographed item, cannot be reused to obtain a latent image of a second or subsequent item without creating double or multiple latent images or latent image distortion on the phosphor screen.
As a result, to reuse the phosphor screen, a process is employed for removing energy and reducing the overall energy level stored in the phosphor screen, sometimes called erasing, at least to a level where the previously stored latent image is no longer readable and no longer detrimentally affects any latent image which is thereafter created on the phosphor screen. In general, the erasing process which is currently employed involves exposing the phosphor screen to infrared radiation at a predetermined intensity and within a prescribed wavelength range (about 1,000 nanometers) for a predetermined period of time. Typically, erasing a phosphor screen in this manner involves exposing the phosphor screen to the infrared radiation for an extended period, typically between 30 minutes and one hour or longer than one hour, the period being determined by several factors, including the amount or intensity of the energy stored within the phosphor screen.
While the above-described erasing process is effective in eliminating or substantially reducing the energy level within a phosphor screen, at least to a level low enough so that the previously stored latent image can no longer detrimentally affect any later created latent image, the erasing process takes an inordinately long time to complete, particularly in view of the much shorter time necessary for creating and reading the latent image. Additionally, the use of infrared energy alone to erase latent images from a phosphor storage screen does not necessarily completely remove the latent image therefrom. Experimental data show that the use of infrared energy tends to lower the energy levels of the screen uniformly so that portions of the screen with latent images as well as non-image areas are all erased linearly. Rescanning and reexamination of the erased screen and its resulting digital image generally reveals some artifacts of the original latent image, thus rendering the screen unacceptable for further imaging. As a result, there is a need for a process for erasing such phosphor screens in a much more efficient, complete, and rapid or accelerated manner.
The present invention, as hereinafter described in greater detail and as illustrated in the attached figures, relates to a method and corresponding apparatus for substantially reducing and/or eliminating energy stored within a phosphor screen in a relatively short period of time, typically on the order of thirty seconds to ten minutes. The use of the present invention permits the process of reading and then erasing a phosphor screen for subsequent use within a time period which is substantially faster than use of the prior art infrared erasing process as described above. Furthermore, the erasing is more uniform and more complete.